#### Doug Kerr

##### Active member

__Direct quantities__

• Aperture (or effective aperture)

• Exposure time ("shutter speed")

• Imaging sensitivity

*(e.g.*, ISO speed)

• Scene luminance

__Indirect quantities__

• Amount of exposure compensation

Of course a change in one of the quantities of "one stop" represents a 2:1 or 1:2 change in the actual underlying quantity (in the case of aperture that being the area, not the diameter).

Now "stop" is not a unit that has an "elegant" background. So in modern times, many authors have tried to replace it with something more "elegant". And sadly, that is often, and mistakenly, the "unit" "Ev".

Ev is not actually a unit; it is a quantity, and is expressed in dimensionless, unitless numbers - just "1", "2", and so forth.

It is a quantity defined by APEX, the Additive System of Photographic Exposure, and expresses, in base-2 logarithmic terms, the joint effect on photographic exposure of a certain aperture (in terms of an f-number) and a certain exposure time. Period. It has no other legitimate use.

So we can say, for example, if our trial shot was a little bit overexposed, we may wish to decrease the photographic exposure by a factor of 1:2 (a "one stop" decrease, in traditional terms); that is,

*increase*the Ev by one unit. Increase? Yes, Ev is defined in that direction, a greater value representing less photographic exposure. But we don't say, "decrease the photographic exposure by one Ev".

Now suppose we just want to decrease our exposure time by a factor of 1:2. Can that properly be expressed as a change of one in the Ev?

Well, if we don't also change the aperture, that is true. But the exposure time itself (independent of its conspiracy with aperture) is properly stated as the quantity Tv, the APEX logarithmic quantity for exposure time. And of course the unit of Tv is not "the TV"; line Ev, Tv is expressed in dimensionless, unitless numbers.

Now we get to the sensitivity of a camera (perhaps the "ISO speed" or the "ISO SOS"). If we double that, have we made a change of "one Ev"? No, for two reasons:

• Ev only applies to photographic exposure, that is, the joint effect of an aperture and an exposure time. It does not apply to camera sensitivity, or the temperature of one's coffee, or the average wage of a photographer's assistant.

• It any case, "the Ev" is not a unit. We can speak of changing exposure, expressed as EV, by one unit, not changing it by "one Ev".

Now what about exposure compensation? On many cameras, the control for this is denominated in "Ev". We may read that if our trial metered image is a little too bright, we might want to apply "an exposure compensation of -1 Ev".

That is not correct, because:

• Ev only applies to photographic exposure, that is, the joint effect of an aperture and an exposure time.

• It any case, the Ev is not a unit. We can speak of changing exposure, expressed as EV, by one unit, not changing it by "one Ev".

Now, it could be argued that, because a one-unit change in EC should lead to a one-unit change in the metered Ev, that it is proper in the case of EC to think of that control as being a control of Ev (on a relative basis). But in any case, a one unit change in Ev (or EC) is not a "one Ev change". It is a change of one in the Ev (or EC)

So, when it comes for a universal "unit" of change in these various quantities, "stop", ragged as its history is, is the only one that is not, in so many cases, just plain "wrong".

Best regards,

Doug